How does the exposure to UV light (wavelength of 200nm) for different lengths of time (0mins, 15mins, 30mins, 45mins, 60mins) affect the concentration of Vitamin C (ascorbic acid) measured through iodine titration

Ascorbic acid, also called vitamin C, is widely known for protecting the body cells from free radicals, playing a role in preventing cancer, chronic heart diseases, and a ray of other diseases. (Chambial et al, 2013) Vitamin C has been used in skin care products since the 1970s. There are multiple benefits of ascorbic acid on the skin, it provides antioxidant properties that reduce the destructive properties of free radicals naturally emitted by the sun. It also promotes collagen synthesis by acting as a catalyst, providing the skin with better elasticity and reducing fine lines. (Doyle, 2023)

The use of vitamin C in skin care products was widely available after Dr. Sheldon Pinnell’s pioneering the formulation of L-ascorbic acid, which is the most stable and active form of vitamin C to retain its effectiveness. He also demonstrated antioxidant properties and its ability to neutralize free radicals. (SkinCeuticals, n.d.) This research led to more use of vitamin C in skin care products.

With the increasing demand for beauty standards and physical aesthetics, the properties of vitamin C help with radiant skin that boosts the self-confidence of individuals. It enhances the positive personal perception and in turn, provides the individual with more positive mental well-being. While healthy skin is not only a matter of physical aesthetics, it improves one's overall health. As mentioned before, ascorbic acid protects the skin from environmental stressors, preventing skin diseases. This reduces the requirement of healthcare costs and improves the quality of life. (Piperberry, 2020)

However, the benefits of vitamin C skin care are still limited to the potential exposure to UV light, which is naturally emitted by the sun’s radiation. The effectiveness of vitamin C is vulnerable during the storage conditions and transportation of the products. The ascorbic acid products may be affected under the sun due to UV rays, making up around 10% of the sunlight exposed to the Earth’s surface. (Lucas, 2017) UV radiation is high-energy radiation emitted at a wavelength of 100-400 nm. (WHO, 2016)

Therefore, this study aims to investigate the effect of direct exposure to UV light on ascorbic acid degradation. A control group of 0 seconds was established and the period of exposure to 200 nm of UV light was set at 900, 1800, 2700 and 3600 at 900 seconds (15-minute) intervals. The final concentration of ascorbic acid will reflect the degradation of ascorbic acid over time.

Ascorbic acid is one of the numerous essential vitamins that the body requires. It is found in natural sources of fruits and vegetables and it is water soluble. It has a formula of C₆ H₈ O₆ . The human body cannot naturally synthesise vitamin C, therefore the ingestion of external sources of vitamin C from supplements or food is required to maintain a constant level of vitamin C in the body for biological processes. (National Institutes for Health, n.d.)

The properties of ascorbic acid allow it to act as an antioxidant. It can act as a reducing agent that undergoes oxidative reaction to produce ascorbate. (FoodCrumbles, 2019) When ascorbic acid is exposed to UV light, it undergoes photodegradation, where it undergoes further one electron oxidative reactions producing ascorbyl free radicals. (Aguilar et al, 2019)

When exposed to UV light, ascorbic acid molecules absorb energy from UV photons. The energy causes the ascorbic acid electrons to move to higher energy levels. Under the excited state of electrons cause the covalent bonds to be broken down, resulting in the formation of free radicals ascorbyl. The free radicals are unstable, making it highly reactive due to the presence of unpaired electrons. The presence of free radicals in the body may lead to oxidative damage. (Dix, 2018) This degradation of ascorbic acid results in ascorbic acid breaking down into smaller compounds that do not hold the same nutrients as the original compound.

To investigate the degradation of ascorbic acid under the exposure of UV light. A redox iodine titration was selected to calculate the final concentration of ascorbic acid after UV light exposure. In the titration, iodine (I₂) solution is the titrant, and starch ((C₆ H₁₀ O₅)_n) acts as the indicator signifying the endpoint of the titration. Iodine reacts with starch to form a dark blue colour from brown, the endpoint of the titration is reached when the analyte turns into a permanent dark blue colour. (Tucker, n.d.) Iodine reacts with ascorbic acid shown in this equation: C₆ H₈ O₆ + I₂ → 2I⁻+C₆ H₆ O₆ + 2H⁺

The starch solution is added to the ascorbic acid solution, the solution is titrated against the iodine solution. The excess iodine reacts with starch after ascorbic acid is oxidized. This forms a dark blue colour indicating the end point of the titration.

“How does the exposure to UV light (wavelength of 200nm) for different lengths of time (0mins, 15mins, 30mins, 45mins, 60mins) affect the concentration of Vitamin C (ascorbic acid) measured through iodine titration ”

Independent variable - The length of time of direct exposure of ascorbic acid solution (conc) to ultraviolet light (200nm) for 0, 900, 1800, 2700, 3600 seconds (± 1s)

Dependent variable - The concentration of ascorbic acid solution (mol dm ⁻³) after exposure to ultraviolet light measured by iodine solution titration

Temperature of the laboratory varying around 23°C is not controlled. The temperature fluctuations should have minimal effect on the results as no windows were opened and the air conditioning was consistent throughout the experiment.

• Weight 0.158g of sodium thiosulfate powder using an electronic weighing scale.
• Put the sodium thiosulfate powder into a 500 cm³ beaker.
• Add 250 cm³ of distilled water into the same beaker.
• Using a clean glass rod, stir the solution until no white powder can be seen.
• Transfer the solution into an airtight glass bottle to prevent deterioration.

• Weight 0.176g of ascorbic acid powder using an electronic weighing scale
• Put the ascorbic acid powder into 100 cm³ beaker
• Add 100 cm³ of distilled water into the same beaker
• Using a clean glass rod, stir the solution until all white powder dissolves in water.
• Transfer the solution into an airtight glass bottle in a dark area avoiding direct light sources to prevent oxidation of the solution.

• Prepare a cardboard box with a lid with a height of more than 25 cm³ , a width of 20, cm³ and a length of 35cm³
• Cut out an opening on the box lid 15cm × 5cm (depending on the size of the UV light)
• Put the UV light on the opening of the lid and tape it down to the lid
• Ensure that when covering the lid over the box, the inside should be completely dark
• Put the beaker containing 0.01 mol dm^-3 of 10 cm³ ascorbic acid solution in the box
• Cover the lid of the box with the UV light
• Switch on the UV light and start the timer for 900 seconds
• Switch off the UV light after 900 seconds
• Remove the beaker of ascorbic acid solution and immediately conduct a titration experiment
• Repeat step 5 to 9 for the different length of time (900, 1800, 2700, 3600 seconds)

• Fill the burette with 0.003 mol dm^-3 iodine solution, the initial volume was recorded on paper
• After 0 seconds, measure 5cm³ of the 0.01 mol dm^-3 ascorbic acid solution using a 10cm³ measuring cylinder and add it to a 50cm³ conical flask
• Measure 5cm³ of distilled water and add it to the same conical flask
• Using a graduated pipette, measure 1cm³ of 1% starch solution and add it to the conical flask
• Put a magnetic stirrer into the conical flask on a hot plate and turn on the stirrer to ensure the mixture will be mixed
• Titrate the solution until the endpoint is reached displaying a permanent trace of a dark blue hue
• Record the final volume of iodine solution on paper
• Repeat steps 2 to 7 for each exposure time (900, 1800, 2700, 3600 seconds)

Raw Data

• Refer to Appendix A for the 5 trials of the volume (cm³ ± 0.05) of 0.003 mol dm⁻³ iodine solution Used to Titrate Ascorbic Acid Solution Until the End Point After Exposure to UV Light For 0, 900, 1800, 2700 and 3600 seconds (±1s) (cm³)

• The colour and smell of the ascorbic acid solution did not change as the time of UV light exposure increases
• No change in colour is observed after the addition of starch solution
• A faint blue is observed for a short time when the iodine solution is dropped into the ascorbic acid starch solution
• The solution turns dark blue after the ascorbic acid starch solution is titrated against the iodine solution after it reaches the endpoint

• The reaction of Iodine with Sodium Thiosulfate solution equation

\(I_2 + 2S_2 O_3^{-2}  → S_4 O_6^{ −2} + 2I^{-}\)

• Concentration of sodium thiosulfate used

 \(number of moles =\frac{mass}{molar\ mass}\)

\(n(Na_2S_2O_3) =\frac{0.158g}{158.11\ gmol^-}=9.99× 10^{-4} moldm^{-3}\)

 \(concentration =\frac{number \ of \ moles}{volume}\)

\([Na_2S_2O_3]= \frac{9.99× 10^{-4}\ moldm^{-3}}{0.250 \ dm^3} =4.00 × 10^{-3} moldm^{-3}\)

• Concentration of iodine
• The titration of 10cm³ of iodine required an average of 15cm³ of sodium thiosulfate
• The molar ratio of sodium thiosulfate to iodine is 2-1 respectively

\(n(Na_2S_2O_3) = 4.00 × 10^{-3} moldm^{-3} × 0.0150dm^3 = 6.00 × 10^{-5} mol\)

\(n(I_2)=\frac{6.00×10^{-5}\ mol }{2.00} =3.00 × 10^{-5}\ mol \)

\([I_2]=\frac{3.00× 10^{-5}mol}{0.01dm^3} =3.00× 10^{-3} mol \ dm^{-3}\)

Therefore in the calculations below 3.00 × 10^-3 mol dm^-3 will be used as the iodine concentration.

Refer to Appendix B for outlier calculations. The calculation of outliers for the volume of 0.003 mol dm^-3 iodine solution used to titrate ascorbic acid solution until the end point after being exposed to −3 UV light for 0, 900, 1800, 2700, and 3600 seconds. The formula below has been applied:

Q1 = first lower quartile

Q3 = third upper quartile

IQR = Q3 − Q1

Lower Fence = Q1 − (1. 5 × IQR)

Upper Fence = Q3 + (1. 5 × IQR)

As seen from Appendix B indicates that there were no outliers for each volume of iodine solution used for different ascorbic acid solutions exposed for different lengths of time in all 5 trials. Therefore all data sets from each trial will be included in the processed data calculations.

\(\text{Average Volume of Iodine Solution Used}=\frac{\sum0.03\ moldm^3\ of \ iodine\ solution\ used \ (cm^3)}{number\ of \ trials}\)

● The molar ratio of iodine to ascorbic acid is 1-1 so the number of moles of iodine is equal to ascorbic acid

• The graph above shows a negative gradient of the trendline, therefore the concentration of ascorbic acid is observed to decrease over the length of exposure to UV light.

The statistical analysis test Pearson correlation coefficient was selected, to indicate the strength of correlation between the length of time exposure to UV light (0, 900, 1800, 2700, 3600 seconds) and the concentration of ascorbic acid solution. All calculations are done with the TI-84 Plus CE calculator.

Figure 14 - Pearson Correlation Formula (Patil, 2023)

\(r=\frac{\sum(x_i-\overline{x})\ (y_i-\overline{y})}{\sqrt{\sum(x_i- \overline{x})^2\sum(y_i-\overline{y})^2}}\)

​ Where,

r = pearson Correlation Coefficient

\(x_i= × variable samples\) 

\(\overline{x}= mean of values in × variable\) 

\(y_i= y variable sample\) 

\(\overline {y}= mean of values in y variable ​\) 

Refer to Appendix C for the calculation of the Pearson correlation coefficient using TI-84 Plus CE. The r-value (Pearson correlation coefficient) was rounded to 3 significant figures showing an r-value of - 0.996. The correlation between the length of time exposure to UV light (0, 900, 1800, 2700, 3600 seconds) and the concentration of ascorbic acid solution was strongly negative.

• The percentage uncertainty of iodine titration calculation The formula used to calculate the percentage uncertainty of the volume of iodine solution used to reach the end point of titration -

\(\text{Percentage uncertainty} =\frac{\sum\frac{uncertainty\ of\ burette}{volume \ of\ iodine}}{number\ of\ trials}×\ 100\%\)

\(\text{Percentage uncertainty}=\frac{uncertainty\ of\ apparatus}{valu \ measure \ by\ apparatus}× 100\%\)

uncertainty of the electronic weighing scale mass of soluble starch + uncertainty of the measuring cylinder volume of water

• Total Percentage Uncertainty This can be calculated by summing up the uncertainty of the volume of iodine solution and uncertainties in Table 8 for each sample.

The absolute uncertainty of the ascorbic acid concentration can be calculated with the formula - 

Absolute uncertainty = concentration of ascorbic acid × total % uncertainty

The experiment was designed to investigate the effect of exposure to UV light on the concentration of ascorbic acid (vitamin C). Through the iodine titration experiment, the investigation of the relationship between exposure to UV light and the concentration of ascorbic acid was possible through calculating the final concentration of ascorbic acid. The results have shown that as the length of UV light exposure increases, the concentration of ascorbic acid decreases. This is due to UV light breaking down ascorbic acid via photodegradation. All samples exposed to UV light (0.009084, 0.008508, 0.007704 and 0.006936 moldm ^-3) of 900, 1800, 2700 and 3600 seconds respectively had lower concentrations than the control group (0 seconds) of 0.009612 moldm⁻³ .

According to Graph 1, the trendline shows a negative trendline over the increased time of UV light exposure. This can be predicted that future values of increasing UV light exposure will result in a decrease in the concentration of ascorbic acid. The Pearson correlation coefficient further strengthens the reliability of the results. The r-value was -0.996, the value is close to -1 indicating a strong negative correlation of the data. Shows that the relationship as the time of exposure to UV light increases, the concentration of ascorbic acid will decrease. Therefore, it is indicated that UV light exposure enhances the rate of degradation of ascorbic acid (vitamin C)

The random error of the experiment was ± 0. 01, which is relatively small indicating a minimal error on the results of the data collected. An error that cannot be accounted for is human error, which is present when monitoring the end-point of the iodine titration by eye. The exact moment of the endpoint of the titration might differ from the other samples due to the change in colour of the ascorbic acid solution.

The use of ascorbic acid (vitamin C) is widely used in food production and skin care. Vitamin C cannot be naturally produced in the human body. Therefore, the results suggest that the packaging of vitamin C is crucial to retain the most nutrients. An opaque bottle that is laminated can decrease sunlight penetration to the solution, the users should avoid exposing vitamin C products to direct sunlight.

Possible extensions to achieve more accurate and precise data for the investigation above. The student could conduct the experiment in a constant and controlled laboratory with constant temperature and light intensity. High-quality apparatus with minimal uncertainty could be used to reduce systematic error. Especially for the set-up of the UV light apparatus, an improvement that could be made is to set up the UV light source carried out in a completely dark room with boarded windows. The UV light source should also be contained in a closed radiation box to minimise the environmental effects and harm towards the human body. Possible extensions of the investigation include testing the method of different packaging effectiveness in preventing photodegradation of ascorbic acid. The samples of ascorbic acid can be exposed to UV light through a filter of the different types of packaging such as glass with different levels of UV coating and matte. Different types of ascorbic acid skin care serums can also be tested for their effectiveness in packaging against the concentration of ascorbic acid in the serum. This can be conducted with the same procedure but the length of exposure to UV light and intensity of the UV light may require preliminary testing to determine its value.

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The Volumes (cm³ ± 0.05) of 0.003 mol dm ⁻³ Iodine Solution Used to Titrate Ascorbic  Acid Solution Until  the End Point After being Exposed to UV Light For 0 seconds (cm³)

Pearson Correlation Coef icient Between The Length of Time Exposure to UV Light (± 1s) (0, 15, 30, 45, 60 min) and the Concentration of Ascorbic Acid Solution (mol dm^-3)